The production of 4 megabit or larger DRAM semiconductors involves the creation of images having submicron dimensions. These are commonly produced by optical lithography using I-line (365 nm) radiation. The resists used in lithographic production of very large scale integration (VLSI) circuits must have sufficient imageability to produce straight walled features in the resist, high resistance to etching of unexposed areas, high contrast, and high operational photospeed. Straight walls are important because sloping walls lead to changes in dimensions and loss of resolution as the resist is eroded by the etching process. Resistance to etching is important for the same reason. High photospeed increases throughput and thereby minimizes the investment in very expensive equipment. High contrast allows tools with lower numerical aperture and larger fields of exposure to be used. Contrast is defined as the ratio of the dissolution rate of the exposed resin (R) to the dissolution rate of the unexposed resin (R.sub.0).
Positive resists are preferred for VLSI. Known I-line photoresist compositions are naphthoquinone diazide sulfonate esters of hydroxyaromatic compounds dispersed in novolak resins as disclosed in U.S. Pat. Nos. 3,130,048 and 3,106,465. Since the function of the unexposed naphthoquinone diazide ester is to inhibit the dissolution of the novolak resin, it is important it be highly miscible with the novolak resin in the film and in solution.
Among the different phenols with which the naphthoquinone diazide sulfonates can be esterified, hydroxy benzophenones and biphenyls have been the most widely used. Molecules that do not contain a benzophenone chromophore are preferred for VLSI applications because the absorbance of benzophenone at 365 nm leads to a residual unbleachable component which can give sloped sidewalls in photoresist images.
4-Sulfonate esters are said to possess advantages for image reversal [G. Buhr, Proc. SPIE 1086, 117(1989)]. In addition, they have the potential to be faster resists than the 5-sulfonates since irradiation produces both a sulfonic acid and a carboxylic acid in the case of the 4-sulfonate, but only a carboxylic acid in the case of the 5-sulfonate. [G. Buhr. Proc. SPIE, 1086, 117(1989); Blum, Proc. SPIE, 771, 148(1987)]. The irradiation products of the 4-sulfonate facilitate the development of the exposed image in simple metal ion free developers (tetramethylammonium hydroxide) which, in the case of 5-sulfonates, usually require surfactants to remove the exposed photoresist completely. The 4-sulfonic acid esters also possess higher absorbance at the I line than the corresponding 5-sulfonates, thus providing higher photospeed and improved internal masking.
Despite the theoretical advantages of the 4-sulfonates, 5-sulfonates remain by far the more popular esters for use in photoresists because of their superior solubility in the novolak resin and casting solvents (U.S. Pat. Nos. 4,266,000 and 4,906,549). For example, it is difficult to achieve more than 15% by weight of solid film with 4-sulfonic acid esters of hydroxybenzophenone. One of the earlier attempts to induce solubility in 4-sulfonic acid esters of benzophenone relied upon leaving free hydroxy groups in di and trihydroxybenzophenones (U.S. Pat. No. 3,130,048). However, the resistance to alkaline developers of compositions containing free hydroxyls is inadequate for practical purposes, especially for VLSI where any loss of unexposed resist results in undesirable, sloping wall profiles. Producing a photoresist that takes advantage of the superior intrinsic characteristics of naphthoquinonediazide 4-sulfonates while maintaining high contrast and good solubility has, until the present invention, remained elusive.
U.S. Pat. No. 4,266,000 discloses naphthoquinone diazide 5-sulfonic acid esters of halogenated benzophenones and bisphenols. Additives to the composition include trihydroxybenzophenone and naphthoquinonediazide 4-sulfonyl chloride. The sulfonyl chloride would be expected to readily hydrolyze to free acid, reducing the shelf life of the resist in solution and introducing the potential for corroding metal surfaces.
U.S. Pat. No. 4,839,256 discloses ink repellent printing plates containing perfluoroalkyl monoethers of dihydroxyaromatics in which the naphthoquinonediazidosulfonate esterfies the other (non-ether) hydroxy site on the benzene ring.
U.S. Pat. No. 4,732,836 discloses photoresist compositions containing tri- and tetrahydroxybenzophenones esterified with mixtures of 4-naphthoquinone diazide sulfonyl chloride and other acid chlorides. Although the generic disclosure would encompass diphenylmethane esters, in fact the single example is of 2,3,4-trihydroxybenzophenone. Moreover, the benzophenone contains no 5-sulfonate esters but rather uses only photochemically inert aliphatic and aromatic acids to block the hydroxyls that are unesterified by 4-sulfonyl chloride. This provides solubility and a lower R.sub.0 but at the expense of photospeed.
U.S. Pat. No. 4,732,837 discloses analogous esters from the 5-sulfonyl chloride.
U.S. Pat. No. 4,873,169 discloses a process for the preparation of low metal content napththoquinonediazide 5-sulfonic acid esters using organic amines as esterification agents.
U.S. Pat. No. 4,906,549 discloses specific phenolic formaldehyde resins for use in conjunction with naphthoquinonediazide 5-sulfonate esters of tri and tetrahydroxy benzophenones in photoresist compositions.
European patent application EP 335,836 discloses positive photoresist compositions containing an alkali soluble resin, a naphthoquinonediazide 5-sulfonate ester with 1,3,5 trihydroxybenzene and an additive of an aromatic hydroxy compound.
U.S. Pat. No. 4,397,937 discloses naphthoquinonediazide 5-sulfonyl esters of unsymmetrical aliphatic diols. Although the generic description includes mixed esters of the diols with 5- and 4-sulfonic acids, no examples of such are disclosed.
U.S. Pat. No. 4,943,511 discloses mono, di- and triesters of 2,3,4-trihydroxybenzophenone with 1,2-naphthoquinone-2-diazo-4-sulfonic acid. Photoresist compositions comprise these esters in combination with particularly chosen cresol formaldehyde resins. The patent demonstrates two approaches (partial esterification and resin choice) that the art has suggested for achieving solubility of 4-sulfonate esters.
There remains a need for a positive working photoresist composition that is based on readily available novolak resins and that provides high photospeed, high contrast, good resistance to etching in unexposed areas, straight-walled profiles, clean development with simple, nonmetallic bases, rapid development in simple, nonmetallic bases, and high solubility in the casting solution.